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diewlasing writes to mention that Oxford scientists have proffered a mathematical answer to the parallel universe question that is gaining some support in the scientific community. "According to quantum mechanics, unobserved particles are described by 'wave functions' representing a set of multiple 'probable' states. When an observer makes a measurement, the particle then settles down into one of these multiple options. The Oxford team, led by Dr. David Deutsch, showed mathematically that the bush-like branching structure created by the universe splitting into parallel versions of itself can explain the probabilistic nature of quantum outcomes."

It's just the Many Worlds interpretation of quantum mechanics, and I don't see anything in the article that's a shocking new revelation about it. The article's just a rehash of an idea that's been around since the 50s.

Commenting in New Scientist magazine, Dr Andy Albrecht, a physicist at the University of California at Davis, said: "This work will go down as one of the most important developments in the history of science."

I would image something that is 'one of the most important developments in the history of science' might qualify as news. Don't you think? Even if proven not to be 'one of the most important' it certainly qualifies for recognition based on that possibility, IMHO.

The problem is that the brief news story was more focused on explaining what the Many Worlds hypothesis is to a lay audience and not really pointing out what the new breakthrough is really all about to a geek audience. Someone needs to link to science site and not a general news site.

Besides being "a physicist at the University of California at Davis", who is Dr Andy Albrecht? and why should I think that he is any more likely than Jack Thompson to recognize one of the most important developments in the history of science? If we are talking Nobel Prize in Physics (or some other prestigious award in the field of physics) winner, maybe there is reason to believe that he is right, otherwise he is just "some dude from California who knows enough to understand the math".

Always hesitate to take people's word for the significance of a piece of work. This is especially true of their own work, naturally, but even friends and colleagues often mis-judge or lack perspective on the importance of a discovery. Furthermore, reporters often misquote or pull quotes from context to the point where they'd be considered falsification if this were a scientific paper. I've even been quoted by a reporter who made the entire quote up from whole cloth. (Seriously, I'd never said anything remotely like what was quoted. Fortunately, it wasn't really a bad quote and I wasn't too bothered, except by the principle.) Thus, when someone says something as hyperbolic-sounding as the quote there, I immediately suspect it.

I *am* a physicist, although I don't have Dr. Albrecht's credentials in this area, so he certainly has a more informed opinion than my own. However, based on my knowledge of the subject, the importance of this finding is in fact fairly over-rated. I don't think that it confirms anything unexpected *and* the theory is, as far as I know, not falsifiable. (I've never heard of a test which would differentiate between the Many Worlds view and the competing interpretations.) So you see, showing that you can't rule Many Worlds out is important, but it does strike me as really revolutionary.

On top of that, the article is a terrible representation of quantum mechanics. It's about time people stopped referring to superposed states as if they were somehow less real than pure states. (And, of course, all the examples are fanciful macroscopic many-worlds ideas.)

But no, it doesn't provide much in the way of actual information, does it?

As described in this article, the factor they claim to have "proved" does indeed make no sense - I was under the impression that the many worlds theory was defined so that this branching tree structure could describe the probabilistic nature, such that this result is a direct consequence of the theory. But I must admit, I'm more of a practical physicist, the minutiae of the underlying explanations for quantum mechanical processes don't really affect me much - is there any kindly passing mathematician who ca

So they say they have a mathematical description of the parallel universe theory. One can construct a mathematical model that describes the geocentric solar system perfectly well, but the the heliocentric version is much simpler.

The probability of the existence of other universes is unrelated to the probability of the existence of parallell universes. There could easily be $BIGNUM universes without any of them being parallel in the Many-Worlds-Interpretation of QM sense.

Of course, it is worth pointing out that speculation about a creator merely pushes the question of origins back a level: where did this hypothetical creator and his/her/their universe come from?

Occam's razor is useless in situations like this. Basically Occam's razor comes down to a judgment call. 'Which "sounds more plausible"? 'is the question that is asked. To be honest when you get into this branch of physics, even things that have been damn well proven and the knowledge used to build workable devices... the concepts STILL sound impossible.

Occam's razor is useless in situations like this. Basically Occam's razor comes down to a judgment call.

Right. Besides which, it's not a "Law Of Nature" anyway -- it's more a rule of thumb. Occam's Razor never "proves" anything. It just lets you make an educated guess as to which avenue of inquiry is, all else being equal (that part's important), more likely to be fruitful. (And thus, which one you'd be wiser to spend the effort to explore.)

(1) (Copenhagen) The act of "observing" a particle at some point between the particle, the measuring apparatus, and your mind, somehow magically causes the particle to collapse from a wave state to a fixed one, without any other action on your part. Nobody has ever explained exactly what an observation is (we are, after all, made of particles too) nor when this happens.

(2) (Multiple worlds) Reality consists of particles in quantum waves of superimposed states. Period. When we observe a particles state, it's state becomes entangled with the state of the particles in our mind, and hence we observe the particle as collapsing to a single state "in each world".

I don't know about everybody else, but the fact that all states can exist, yet I can only perceive them separately, is no stranger to me than that all moments of time exist, yet I can only perceive each one separately.

Reality consists of particles in quantum waves of superimposed states. Period. When we observe a particles state, it's state becomes entangled with the state of the particles in our mind, and hence we observe the particle as collapsing to a single state "in each world".

Many people mix those up. Our "mind" doesn't have anything to do with it.

The wave function collapses when a particle interacts with a macrosystem. When two macrosystems are separate from each other, we have to assume the other macrosystem is not coherent with us until contact.

And remember contact *includes* photons reflected off the other macrosystem, so if we see it, otherwise put "observe" it, we're already in contact.

I do not see anything new here at all. The "many worlds" hypothesis has always depended on a hypothetical "probability tree" to describe the odds of quantum occurrences. This idea was new to me, oh, about maybe 30 years ago, and was not actually new then.

Are they trying to claim that their mathematical probability tree corresponds to a "real" probability tree? If so, on what basis do they make that claim?

To them, I say: "Show me evidence, and I will believe. Until then, stop bothering me with old idea

This is a fairly subtle point, so I'm not sure that I'm going to explain myself properly... but here's my best shot:

The Many-Worlds [wikipedia.org] concept of quantum mechanics was originally presented as an interpretation of the theory. It was viewed by many as being ridiculous, or "non-economical with universes" as the joke goes. Work in fields like quantum decoherence [wikipedia.org] has, over the last few decades, helped to explain how "normal" (classical) states emerge from quantum superpositions. Decoherence, briefly, explains how a superposition of quantum states evolves deterministically (no randomness!) into a discrete set of pseudo-classical states (due to entanglement with the many degrees of freedom available in the "environment"--i.e. the universe at large). This extension to quantum mechanics has been tested experimentally and verified.

The remaining issue in a theory of quantum + decoherence is that the classical states have the right probabilities, but there is still nothing to explain why we observe a particular classical state (photon measured spin-up instead of spin-down). However the (ad-hoc) postulate of wavefunction collapse, no longer being necessary to explain how the probabilities arise, can in fact be entirely removed if we allow that the global superposition never collapses.

Thus, a local observer (e.g. an instrument or a human) perceives a single outcome only because they are a participant in this "global superposition" (the superposition of the entire universe). The wavefunction of the universe as a whole evolves deterministically.

Okay, that was a long-winded preamble, and I still have not answered your question. The answer is that the existence of multiple universes cannot be falsified per se. But, then again, in this formalism Many-Worlds is not an axiom: it is a prediction. Given that it is a prediction of a thoroughly successful theory, we should be compelled to accept the prediction as correct even if we cannot directly test it. We can, at least, test other predictions of the theory. In principle, we can test for superpositions as big as we like (superpositions of entire galaxies, etc.), but we cannot ever test that final prediction: that the universe as a whole is also in a superposition. But, if we've tested the theory in every other way, can we really "throw away" the final prediction about the global superposition?

Now, I know many of you will counter-argue that non-falsifiable predictions are not science, and should be ignored as metaphysics, or even "meaningless." Perhaps. But allow me to draw an analogy: One of the fundamental assumptions of science is that there is such a thing as "physical law." That is, we can extrapolate from one measurement to others. Put otherwise, we accept that the laws of physics are the same here as they are in a distant galaxy. Note that, because of the expansion of the universe and the speed-of-light-limit, there are some regions of the universe that we cannot ever explore (even in principle, assuming our current physics is correct). Thus, the prediction that "the laws of physics are invariant across the universe" is itself unfalsifiable, yet we generally accept it to be true.

Similarly, we need but extend this logic into quantum mechanics, where if assume that the laws of physics are the same everywhere in the universe (and everywhere within the wavefunction of the universe), then we should accept that the global superposition is probably correct: i.e.: Many Worlds "exist" (but are inaccessible to us). I agree that this conclusion is uncomfortable, but it appears inescapable given our current understanding of physics. (Note: As a scientist I'm of course allowing for the possibility of future measurements disproving some part of this logic--this is entirely based on our current understanding.)

As I said, the point I'm trying to make is not obvious. Hopefully I've not muddled it beyond understanding.

For anyone interested, this argument was made much more clearly than I am able to in a recent Nature review article:
Max Tegmark. "Many lives in many worlds [nature.com]" Nature 448, 23-24 (5 July 2007) | doi:10.1038/448023a [doi.org]; Published online 4 July 2007.
The blurb is:

Accepting quantum physics to be universally true, argues Max Tegmark, means that you should also believe in parallel universes.

The article is only available to subscribers, but here are some quotes from the article:

I have another question, which might sound kind of naive. If we can accept that there are many universes, with new ones sprouting all the time, is there some constraint on how those universes are? That there might be an infinite set within a certain limit? Otherwise, if, in one of these other universes, the laws of physics are different, then those inhabitants or observers might be able to travel between universes -- just like in another universe where the speed of light follows different laws, they might b

If we can accept that there are many universes, with new ones sprouting all the time, is there some constraint on how those universes are? That there might be an infinite set within a certain limit?

Short answer: there are constraints and limits.

Long answer:
First off, some of the discussion here is getting confused because people are equating the "many universes" of the Many-Worlds Interpretation with the "parallel realities" espoused by other theories (but, most prominently displayed in sci-fi), where

That the laws of physics are invariant (first explicitly stated in geology as uniformitarianism [wikipedia.org]) is not a prediction, but rather an axiom of science.

Agreed. It is an axiom of science as we do it.

With this understanding the status of many worlds and the absurdly large number of parallel universes it entails is quite different from uniformity of natural laws. One (spatiotemporally uniform natural laws) is an assumption necessary for the scientific enterprise to function at all. The other (la

The many worlds interpretation of quantum mechanics doesn't fit a primary requirement which is that it be falsifiable. We need to be able to do an experiment that would have a different outcome of the hypothesis were true or false. As far as I know, nobody has thought of any such experiment yet. The many world interpretation is a philosophical debate, but not yet a scientific one.

OTOH, dark matter and dark energy demonstrably do exist. There are repeatable experiments that show

saying there must be some undetectable, magical force acting on all the matter in the universe because the calculations we've come up with so far are inaccurate strikes me as lazy and uncreative.

You don't think that scientists are working to figure out a way to detect them? Currently, dark matter and energy are placeholders that people use while they characterize the anomalies that brought the concepts about. If we assume that *something* causes the anomalies, then we can study it until we know enough ab

And the whole dark matter/dark energy thing strikes me as a load of humbug... saying there must be some undetectable, magical force acting on all the matter in the universe because the calculations we've come up with so far are inaccurate strikes me as lazy and uncreative.

It worked once before. Calculations of the orbit of the planet Uranus were noticeably inaccurate; the planet wasn't quite where it ought to have been. One explanation was that this was the result of the gravitational effect of a large amount of dark matter. This dark matter was later found and named Neptune.

It also failed to work once before. Calculations of the orbit of the planet Mercury were noticeably inaccurate; the planet wasn't quite where it ought to have been. One explanation was that this was the result of the gravitational effect of the planet Vulcan, which orbited so close to the Sun that it was practically impossible to observe. Then relativity came out, and the new calculations were accurate.

Yeah and the time difference between "I think it works like this" and "See we can prove it now" grows increasingly large the more complex the subject matter. You realize that our understanding of things is still limited by our vantage point in the universe and that we DON'T actually understand all the workings many claim to. Quantum anything is a pretty good example of one of those monkey wrenches of "AHA bitches, and you thought you understood how this works!, Gotcha" The part that is Science is the "Da

From my perspective, even if this mathematical "proof" is true, it is only true in the ontological sense, i.e. that these branches can happen and maybe do happen, but not in reality. Then again, I believe the entire basis for the universe is ultimately ontological but that's a different matter.

My point is that these alternate "universes" may only exist in infinitesimally-small times (possibly below the Planck time threshold) and then simply cease to exist again as compared with our reality in the next moment, moment after moment.

I think what it is saying is our universe split like cells, at LEAST at every point you make a quantum measurement. You don't exist in a single universe, per se, but rather a long string of universes you used to be in, up to the point of the one you are in now.

Perhaps you're asking the exact same question in a whole bunch of other universes...

Maybe since once a quantum "decision" is made, there is no more communication possible between the different states. Our consciousness rides along both paths, but has no way of knowing about the others.

My theory is that we actually ARE experiencing parallel universes. But the pressures of biological evolution have driven us toward brain structures which hide this fact. Maybe we actually ARE spread out across many different possibilities, but our conscious view of reality is as a single whole. Why? Because it made survival easier, perhaps. Or maybe, individuals with parallel awareness inevitably go insane and die out. Who knows.

Sometimes this parallelism "leaks through," in the form of quantum strangenes

I'm not sure if you were already aware, but there is indeed a concept called "Quantum Darwinism [wikipedia.org]" which helps explain (using the results from quantum decoherence [wikipedia.org]) why we observe things "classically" (single outcomes of experiments, non-entangled macroscopic states, etc.) despite the universe being fundamentally quantum.

Briefly, the theory shows (rigorously) how pseudo-classical states are the only ones that are robust against decoherence. Hence, those are the states that tend to persists for measurable periods of time. And those pseudo-classical states are the ones that give rise to other pseudo-classical states.

Moreover the main developer of these ideas (Wojciech Zurek [wikipedia.org]) describes in his papers how what we typically term "memories" are inherently classical states (it's either "a" or "b"--not a superposition of both). He explains how macroscopic states will tend to be pseudo-classical, so of course any biological (macroscopic) creature will evolve to assume that reality is classical (it's an adaptive advantage and a good approximation of reality).

The point is that these larger-scale superpositions do indeed exist, but that local observers (e.g. instruments, or ants, or humans) can inherently only record/remember classical states, not quantum ones. So, our perception of reality (and memory of reality) is inherently a classical one.

There is a circular dependency here. The author assumes that the parallel universe interpretation is correct and then argues that if this interpretation is correct, then we can derive probabilistic nature of quantum of mechanics. All this means is that the parallel universe is a self-consistent theory. Nobody has argued against this for the last 50+ years.

The problem with quantum mechanics interpretation is that as of now, no interpretation exists which is not bizarre in our traditional world view. Parallel universe is just one of them.

Where does the energy come from to give existence to this second universe? This whole splitting of the universe thing seems common in physics, so I'm sure I'm not interpreting this correctly. It seems like there's entire universes being created because of the uncertainty of a single particle.

....showed mathematically that the bush-like branching structure created by the universe splitting into parallel versions of itself can explain the probabilistic nature of quantum outcomes

Or in other words, this science fiction nonsense about parallel worlds, unscientific because it can never be tested or proven, and which was inspired by observations of quantum mechanics, is now supposedly able to explain, guess what,... quantum mechanics, the very concept that the nonsense was built on in the first place.

The absurd number of parallel universes that would have to be created is mind boggling, since, at the very least, an entire universe would have to be created every single time any atom decayed (one for the universe where that atom happened to decay at that instant, another for the case where that atom didn't happen to decay). Strange that none of the wackos who advocate this, and I use the term very loosely, "theory", bother to expain where all of the mass and energy is coming from for all of these extra universes. Note that we are talking about far more universes than atoms in our own universe. Absolute hogwash.

My question would be, are the number of parallel universes countably infinite (same cardinality as the integers) or uncountably infinite (same cardinality as the real numbers)? If countable, this suggests that the number of quantum potential states in the universe are countable, and would seem to lend credence to the idea of an orderly deterministic universe. If uncountable, then the multiverse is infinitely deep - more satisfying, perhaps, in a religious worldview.Unless, of course, God(s)(ess)(esses) cons

Strange that none of the wackos who advocate this, and I use the term very loosely, "theory", bother to expain where all of the mass and energy is coming from for all of these extra universes.

The mass and energy isn't coming from anywhere, because there's no new particles being created. The particles are the same ones, in all universes, their state is just getting more complex, and each "parallel universe" is just a description of one consistent state of all the particles of the universe over all histories. We only observe the particles as as having measurable (subject to Heisenberg) positions and velocities because we're using other particles to measure what those positions are.

A better question might be "where is the information needed to describe the state of the particle stored". Or to put it another way "how many bits does God's Computer have, and can we hack it?"

The absurd number of parallel universes that would have to be created is mind boggling, since, at the very least, an entire universe would have to be created every single time any atom decayed (one for the universe where that atom happened to decay at that instant, another for the case where that atom didn't happen to decay).

What exactly is the absurdity scale you are using to measure the absurdity of this idea? Four parallel universes are okay, six are goofy, seven are silly, 10 are ridiculous, and 1000 or more are absurd?

Strange that none of the wackos who advocate this, and I use the term very loosely, "theory", bother to expain where all of the mass and energy is coming from for all of these extra universes.

That's like asking where the mass and energy in our universe came from. It's the same answer in all parallel universes -- it was there all along. When they talk about a new universe branching, it's not a big bang event, where a new universe is born, it's an altered copy its twin, identical up until the point where the quantum decision was made. It has a completely identical history after a certain point and therefore, the same mass and energy. Parallel universes do not 'share' energy, nor information, nor anything else. They don't 'seed' or 'give birth to' each other. They are totally out of contact with each other. We wouldn't even know about other ones, if not for the math.

Note that we are talking about far more universes than atoms in our own universe. Absolute hogwash.

I can't see why anyone modded you insightful here. You seem to be arguing from personal incredulity [wikipedia.org]. Not that I'm claiming that these guys are right or their theory is true, but your skepticism seems more emotional than rational to me.

There is a great quote by physicist Max Tegmark: "The critique of many worlds is shifting from 'it makes no sense and I hate it' to simply 'I hate it'."

As far as the meat of it, traditionally the Many-Worlds Interpretation has had two technical objections raised. The first is called the basis problem, and the second is deriving correct probabilities. The basis problem is that when the universe "splits" it's not clear how it should split. The math allows for infinite different ways to split, but we only see one way. This has been solved in recent years by the study of decoherence, which in MWI terms is like looking at the splitting process up close. Turns out it can only happen one way in practice. So that one's done.

The article is more about the other one, deriving probabilities. Actually it's easy to derive probabilities in the MWI, but they're wrong. The right probabilities are what is called the Born rule, and it's been hard to get those. David Deutsch came up with a new idea in 1999 where he proposed tying it in to decision theory. He said that we really care about probabilities because they influence how we make decisions about what to do. If we can derive a reasonable decision theory within the MWI, then we've essentially explained probabilities. His work had some shortcomings but subsequent efforts have largely resolved those.

So now for the first time, the two traditional technical problems with the MWI have reasonably good solutions. Hence we are back to, as Tegmark says, "I hate it" as the main objection to the theory. Since that's not really a good argument, it can be said that the MWI should be considered the most compelling candidate for an interpretation of QM.

One final link, here is one of the papers that extends Deutsch's idea about decision theory and pretty much closes the holes: http://arxiv.org/abs/quant-ph/0312157 [arxiv.org]. It's pretty technical but still a lot more readable than most physics papers.

I abhor mathematical proofs of supposedly scientific things. Math is a tool for constructing human thought in meaningful exchanges for other humans to understand. It is, therein, a language and not a scientific form of proof in and of itself.

As just one more language, math suffers the faults that any language can suffer. Just because something makes a working equation does not give it validity. Certainly no more than when a sentence is grammatically correct does that make the sentence a proof of anythi

We, in the fire department (when driving our elevated platform aroudn town), use the phrase, "A miss is as good as a mile." Especially as an answer to the question, "Did you come close to that telephone-poll/car/sign?":P

Not really. It's convetionally interpreted as "It was a miss, but a near thing." And convention generally trumps nitpickers.:)You *can* use "near hit" and I believe the safety industry actually uses the two interchangeably. It might be one of those glass half full or half empty situations.

But it boils down to what it really was. Was it a hit or a miss, and what kind of hit or miss.

Here's how *I* would put it:Near miss: Objects miss, but it was close.Near hit: Objects collided in glancing blow.Distant miss

I did... and here's my solution. Basically, there are other branches where life sucks less, and others where life suck more, etc. I'm going to find the one where life sucks less and kill my alter ego so I can take their place!!!!!!!! (Of course, that means that there's ANOTHER me in an even suckier universe gunning for me, so better be quick!)

You'd better watch out - there may be a doppelganger of you looking to do the same thing...

Life is like a dream. You never die in your dreams. You never die in your observed life. You just die in alternate universes. This carries on until you reach the next plane of intelligence (wake up) whereupon you realize that there was no mind-body problem to begin with.

It's not obvious that anything will happen, just thanks to the mulitple-words interpretation. Some things actually have probability zero.

Now, I'm not well-versed enough to say anything substantial about spontaneous particle pairs appearing and fixing things, but I think you really need to consider the possibility that at some point (maybe when you were born, maybe yesterday, maybe tomorrow) life according to the path you've experienced so far will, inevitably, lead to your death. Not "almost surely" (limes

You honestly think they waited for this to be peer-reviewed and published before they belted out the press releases? That would mean they may be expected to provide some detail, which would be madness!

Seriously though, there's no sign of a citation from any of the people running the story (most of which are nearly identical, so they're probably just copying from the same press release), and there's no sign of it on arXiv or from a quick trawl of journal feeds, so it's a very good chance that it's either unpublished work, or a conference paper somewhere. Not surprising, given how many "most significant discoveries in the history of science" turn out to be much less dramatic under the cold hard light of review than when they're first reported.

Schroedinger's cat may or may not have died of old age by now, depending on whether or not it survived the initial box/gassing experience. Somewhere in Vienna near Schroedinger's house, there is a gravesite which may or may not contain cat bones.